The present invention relates to an image forming apparatus for forming images from image information of originals placed on the original table and, more particularly, to an image forming apparatus wherein a reading unit to read the image information is moved relative to the original table.
This type apparatus, for example, a copying machine is provided with the original table fixed to the machine body and an exposing unit arranged to reciprocate along the plane of this original table. Copying is done in such a way that the exposing unit which includes an exposing lamp, mirror and the like is reciprocated along the fixed original table to expose and scan the originals on the original table. Since the originals to be copied have various sizes such as A4 and B5, sheets of paper on which images are to be formed are grouped according to these original sizes and are previously set in the copying machine.
However, it happens sometimes that an original of A4 size, for example, is copied on sheets of B5 size, for example, because the size of these sheets which have been previously set in the copying machine is not recognized, or that the original is set, long side up, on the original table in spite of setting it short side up, even when the original and sheets are the same (A5 size, for example) in size. In the case of the conventional apparatus, therefore, the area where copying can be made cannot be known in the stage of setting the original on the original table, thereby causing incomplete image formation (or copying). Particularly when the original is not of standard size, the above-mentioned copying mistakes are liable to be caused.